The present invention is based on a sensing element for determining the concentration of oxidizable constituents in a gas mixture, in particular for the determination of saturated and unsaturated hydrocarbons, nitrogen oxides, and ammonia.
Sensing elements which contain sensor materials of the general formula A2xe2x88x92xAxe2x80x2xBO4 are known from German Patent 23 34 044 C3. These are rare-earth compounds of the K2MgF4 structural type, for the detection of oxidizable gases. It is also known from German Patent 42 44 723 A1 to use rare-earth cuprates of the formula A2xe2x88x92xLxCuO4 for the detection of oxygen in gas mixtures, in particular in exhaust gases of internal combustion engines and combustion facilities. In addition, the article by H. Meixner and U. Lampe in Sensors and Actuators B 1996, 33, pp. 198-202 describes a plurality of metal oxides for the determination of various gas components. For some time, however, it has proven difficult to find suitable materials with high selectivities for, e.g., the determination of saturated and unsaturated hydrocarbons and of ammonia or nitrogen oxides. This was attributable, among other factors, to the poor corrosion stability of the electrode materials used, which often have a strong tendency toward undesirable sulfate formation on the electrode surface.
In contrast to the known existing art, the sensing element according to the present invention having the material used for the measurement electrode, which contains either an electrically conductive spinel of the general formula ABBxe2x80x2O4 or an electrically conductive pseudobrookite of the general formula ABBxe2x80x2O5, exhibits excellent corrosion resistance at high temperatures, a low tendency to form sulfates, and a high selectivity for oxidizable gaseous compounds. It is thus possible, in a simple and advantageous manner, to determine corrosive ammonia without having the measurement electrode chemically attacked. As a result of the possible structural variations of these two classes of compounds, i.e. the spinels and the pseudobrookites, it is possible to make available a variety of structures for the determination of various gases. Because of their structure, spinels possess a particularly high degree of spatial occupancy, in which one-eighth of the tetrahedral vacancies and half the octahedral vacancies of the oxygen sublattice, which forms approximately a cubic close-packed (ccp) configuration, are occupied by cations. This dense structure, which is also exhibited by the pseudobrookites, inhibits or prevents any diffusion of the metal cations which contaminate the electrode material, derived for example from oxides of exhaust systems, into the metal oxide electrode; this would be associated with poisoning of the electrode and thus with a change in signal.
The features set forth in the dependent claims make possible advantageous developments of and improvements to the sensing element recited in the principal claim.
In a preferred embodiment, what is used as the spinel is a so-called 2,3 spinel, A representing a divalent transition metal cation and B and Bxe2x80x2 a trivalent transition metal cation. A can, for example, represent the divalent cations of cobalt, nickel, or copper, B the trivalent cations of chromium, iron, and manganese, and Bxe2x80x2 the trivalent cations of chromium and manganese. 2,3 spinels, for example NiFeMnO4 or CoCr2O4 or CoCrMnO4, have a high sensitivity in particular for unsaturated hydrocarbons. The oxygen content of the gas mixture, as long as it exceeds 1% in, for example, exhaust gases of internal combustion engines, has no great effect on the selectivity or sensitivity of the measurement signal; the same is true for other gas constituents. The measurement electrode has a thickness of 5-100 xcexcm, preferably 20-30 xcexcm.
In a further advantageous embodiment, what is used as the spinel is a 4,2 spinel, A representing a tetravalent transition metal cation and B and Bxe2x80x2 a divalent transition metal cation. A can be, for example, a tetravalent cation of titanium or zirconium, but niobium is also possible. B and Bxe2x80x2 can represent, for example, the divalent cations of cobalt and nickel. One possible combination is, for example, TiCo2O4, so that particularly high sensitivities for nitrogen oxides can be attained with this metal oxide electrode. The sensitivity of these 4,2 spinels for nitrogen oxides is so high that the other constituents of the exhaust gas exhibit no cross-sensitivities. The oxygen content of the lean exhaust gas also has no great influence on the sensitivity of the sensor signal.
A further advantageous embodiment of the sensing element according to the present invention consists in the use of so-called 6,1 spinels, A representing a hexavalent transition metal cation and B and Bxe2x80x2 a univalent metal cation. A can be, for example, the hexavalent cation of tungsten, molybdenum, or chromium, and B and Bxe2x80x2 represent, for example, the univalent cations of the coinage metals and of elements of the first main group, for example gold, silver, copper, potassium, lithium, and sodium. Depending on their composition, 6,1 spinels exhibit high sensitivity for a variety of gases, so that with suitable combinations of 6,1 spinels both hydrocarbons and nitrogen oxides, or also ammonia, can be determined.
In a further preferred embodiment, a pseudobrookite of the formula ABBxe2x80x2O5, composed of metallic transition elements, can be used as the sensor material. In this context A represents a tetravalent transition metal cation, and B and Bxe2x80x2 a trivalent transition metal cation. In particular A represents, for example, the tetravalent cations of titanium and zirconium, niobium also being possible. B and Bxe2x80x2 represent the trivalent cations of chromium, iron, and manganese. Pseudobrookites according to the present invention have a high sensitivity for saturated and unsaturated hydrocarbons; once again, no cross-sensitivities to the other constituents in a gas mixture occur. The sensing element is also characterized in that occupancy of the positions B and Bxe2x80x2 is accomplished stoichiometrically or nonstoichiometrically, and correspond to the general empirical formulas ABxBxe2x80x22xe2x88x92xO4 and/or ABxBxe2x80x22xe2x88x92xO5, where 0 less than x less than 2.